1. Field of the Invention
The present invention relates to an image forming apparatus that uses an electro-photographic process, and more particularly, to a technique for transmitting a horizontal synchronization signal used for the electro-photographic process.
2. Description of the Related Art
FIG. 13 illustrates a circuit diagram of an input/output interface between a laser illumination signal (image signal) and a horizontal synchronization signal (hereinafter sometimes referred to as “BD signal”) according to a related art. The laser illumination signal (image signal) and the horizontal synchronization signal (BD signal) are typically connected to individual signal 0lines, and the construction is discussed in Japanese Patent Application Laid-Open No. 2000-316076, for example.
A video controller 440 performs a role of receiving image information from a host computer to convert the image information into an image signal (VDO1). The video controller 440 includes a buffer 105 that receives a BD signal, a pull-up resistor 139, and a driver 101 that converts the image signal (VDO1) into a differential signal. The video controller 440 also includes output impedance matching resistors 131 and 132 and a connector 151.
A laser driver board 120 functions as a laser driver unit that emits a laser beam. The laser driver board 120 includes a laser driver 111 that drives a semiconductor laser 421, a receiver 102 that receives differential video signals (VD1 and /VD1) as input, an input impedance matching resistor 135, a BD IC 113 that functions as a horizontal synchronization signal detector, a protection resistor 137, and a connector 209. Here, the symbol “/” represents a negative logic (low-active).
A DC controller 201 is a control unit of a printer engine. The printer engine includes various elements for forming color images which will be described in detail later with reference to FIG. 2.
The DC controller 201 has connectors 152 and 210 that link the video controller 440 and the laser driver board 120 together. A flexible flat cable (FFC) 211 is used for connecting the DC controller 201 and the laser driver board 120. The connectors 151 and 152 are general board-to-board connectors which are compliant with the DIN standards (IEC603-2/DIN41612 standards).
FIG. 14 illustrates a timing chart of the circuit diagram of FIG. 13.
The BD signal (/BD1) used as the horizontal synchronization signal has a negative logic and becomes a low level “L” when a laser beam L is being irradiated to a BD sensor 160. In the drawing, VD1 and /VD1 respectively represent positive and negative logic outputs of the driver 101. Both of the outputs have the lowest voltage level of 0.77 V for “L” and the highest voltage level of 1.77 V for “H”. When the BD signal (/BD1) used as the horizontal synchronization signal becomes “L”, the video controller 440 outputs the image signal (VDO1) at an image area after a predetermined period. Thereafter, by the known electro-photographic process, images are developed on a photosensitive drum and transferred to recording sheet.
The recording sheet is subsequently heated and pressurized so that the images are fixed.
In practice, in addition to the elements illustrated in FIG. 13, the laser driver board 120 includes a signal line for supplying electrical power to each part thereof. In addition, the laser driver board 120 includes a signal line for transmitting a command that causes the semiconductor laser 421 to illuminate laser light compulsorily in order to detect the BD signal and a PWM signal line for controlling the emission time of the semiconductor laser 421. Such signal lines are not depicted for the sake of clarity.
In the case of a color laser printer, such signal lines must be provided for each color of yellow (Y), magenta (M), cyan (C), and black (B). That is to say, signal lines for the BD signals of each color which are output from the laser driver board 120 and input to the video controller 440 must be provided between the video controller 440 and the laser driver board 120. Moreover, signal lines for the differential video signals of each color which are output from the video controller 440 and input to the laser driver board 120 must be provided between them. Therefore, four times signal lines of three signal lines illustrated in FIG. 13 are necessary, namely, twelve signal lines are required in total.
Particularly, when the ends of signal lines are configured by connectors, the connectors need to have a size corresponding with the number of signal lines, which leads to an increase in the connector costs.
Therefore, it is desirable to reduce the number of signal lines without sacrificing the functions related to the horizontal synchronization signal.